Progressively varying focal power opthalmic lens

A progressively variable focal power lens, said lens having a progressive power surface comprising a distance portion, a reading portion, a progressive corridor and peripheral portions, whereby at least a portion of said surface can be defined in parametric form with reference to a three-dimensional coordinate system in which the origin of said coordinate system is at the vertex of the lens surface, the z axis is perpendicular to said surface at the vertex, the x axis is vertical relative to said surface, and the y axis is horizontal relative to said surface, wherein each cross-section of said portion parallel to the yz plane is defined as a two-dimensional curve having a central portion defined by the parametric function Z.sub.m(x,y) and peripheral portions defined by the function Z.sub.n(x,y), and wherein:

• ##EQU5## and ##EQU6## where:
• Z.sub.mo(x) is the function that describes the geometry of the umbilical line of the lens surface;
• B.sub.(x) is the conic constant for the conic component of the function;
• .rho..sub.(x) is the y,z component of local radius of curvature R.sub.m(x) of the umbilical line defined by the function Z.sub.mo(x) ;
• C.sub.(x) is a very small parameter to suppress the contribution of the exponential component,
• 1/n.sub.(x) C.sub.(x) y.sup.n (x), in the central portion of the curve;
• n.sub.(x) is the parameter responsible for the smooth connection of the curve Z.sub.m(x,y) to the curve Z.sub.n(x,y), the value of n.sub.(x) being found by solving the equation ##EQU7## for the values of y equal to the points of connection;
• ##EQU8## wherein:
• B.sub.L and R.sub.L are chosen to provide the desired width for the reading portion of the lens surface, R.sub.p(x) =R.sub.(x) (1-A.sub.(x))
• where R.sub.(x) equals the distance between a point x.sub.i y.sub.i on the curve and the line parallel to the x axis and intersecting the center of curvature of the distance portion of the lens surface;
• and
• A.sub.(x) is a modifying factor for correcting the distortion in horizontal and vertical lines in the temple side of the peripheral portion after rotation of the lens to accommodate for left or right eye.